Association is nil when model accepts_nested_attributes

nested_attrib_test.rb

begin
  require 'bundler/inline'
rescue LoadError => e
  $stderr.puts 'Bundler version 1.10 or later is required. Please update your Bundler'
  raise e
end

gemfile(true) do
  source 'https://rubygems.org'
  # Activate the gem you are reporting the issue against.
  gem 'activerecord', '4.2.2'
  gem 'sqlite3'
end

require 'active_record'
require 'minitest/autorun'
require 'logger'

# Ensure backward compatibility with Minitest 4
Minitest::Test = MiniTest::Unit::TestCase unless defined?(Minitest::Test)

# This connection will do for database-independent bug reports.
ActiveRecord::Base.establish_connection(adapter: 'sqlite3', database: ':memory:')
ActiveRecord::Base.logger = Logger.new(STDOUT)

ActiveRecord::Schema.define do
  create_table :cars do |t|
  end

  create_table :doors do |t|
    t.references :car
  end

  create_table :wheels do |t|
    t.references :car
  end

  create_table :seats do |t|
    t.references :car
  end
end

class Car < ActiveRecord::Base
  has_many :doors

  has_many :wheels
  accepts_nested_attributes_for :wheels

  has_many :seats, inverse_of: :car
  accepts_nested_attributes_for :seats
end

class Door < ActiveRecord::Base
  belongs_to :car
end

class Wheel < ActiveRecord::Base
  belongs_to :car
end

class Seat < ActiveRecord::Base
  belongs_to :car, inverse_of: :seats
end

class CarTest < Minitest::Test
  def test_assoc_without_nested_attribs
    c = Car.new
    d = c.doors.build
    assert_equal c, d.car
  end

  def test_assoc_with_nested_attribs
    c = Car.new
    w = c.wheels.build
    assert_equal c, w.car
  end

  def test_assoc_with_nested_attribs_and_inverse_of
    c = Car.new
    s = c.seats.build
    assert_equal c, s.car
  end
end